1. Field of the Invention
This invention relates to microwave dual reflector antennas typically used in terrestrial point to point, and point to multipoint applications. More particularly, the invention provides a low cost self supported feed solution for use in frequency bands between 5 GHz and 60 GHz wherein stringent regulatory standard compliance and or specific system electrical characteristics are required. The invention is particularly suited to “deep dish” designs overcoming performance limitations of prior art devices and obviating the need for a conventional shroud assembly. It is also applicable to more conventional dish profiles.
2. Description of Related Art
Dual reflector antennas employing self-supported feed direct a signal incident on the main reflector onto a sub-reflector mounted adjacent to the focal region of the main reflector, which in turn directs the signal into a waveguide transmission line typically via a feed horn or aperture to the first stage of a receiver. When the dual reflector antenna is used to transmit a signal, the signals travel from the last stage of the transmitter system, via the waveguide, to the feed aperture, sub-reflector, and main reflector to free space.
Dual reflector antennas utilizing a sub-reflector supported and fed by a waveguide are relatively cost efficient. This configuration also facilitates the mounting of an “Outdoor Unit” comprising the initial stages of a transceiver system, directly onto the back of the main reflector and also eliminates the need for a separate feed support structure that would conventionally span the face of the main reflector, thereby introducing some loss in operating efficiency. The waveguide can have either a rectangular cross-section, whereby the antenna is single polarized, or can have a square or circular cross-section facilitating dual-polarization operation.
The electrical performance of an antenna used in terrestrial communications is characterized by its gain, radiation pattern, cross-polarization and return loss performance efficient gain, radiation pattern and cross-polarization characteristics are essential for efficient microwave link planning and coordination, whilst a good return loss is necessary for efficient radio operation.
These principal characteristics are determined by a feed system designed in conjunction with the main reflector profile. Conventional antenna designs used extensively in terrestrial point to point communications utilize a parabolic main reflector together with either a “J-hook” type waveguide feed system, or a self supported sub-reflector type feed system. In order to achieve “high performance” radiation pattern characteristics, these designs typically use an RF energy absorber lined cylindrical shroud around the outer edge of the main reflector antenna in order to improve the radiation pattern particularly in directions from approximately 50 to 180 degrees from the forward on axis direction. Shrouds however increase the overall weight, wind load, structural support and manufacturing costs of the antenna.
An alternative method to improve the radiation pattern in these angular regions is to use a “deep” dish reflector, i.e. the ratio of the reflector focal length (F) to reflector diameter (D) is made less than or equal to 0.25 (as opposed to an F/D of 0.35 typically found in more conventional dish designs). Such designs can achieve “high performance” radiation pattern characteristics without the need for a separate shroud assembly when used with a carefully designed feed system which provides controlled dish illumination, particularly toward the edge of the dish. One such design which uses corrugations proximate to the outer radius of the sub-reflector to inhibit surface propagation and or field diffraction around the outer edge of the sub-reflector is described in U.S. Pat. No. 5,959,590 issued Sep. 28, 1999 to Sandford et al.
In dual-reflector feeds employing dielectric cone supported sub-reflectors, adequate feed radiation pattern characteristics may be designed for conventional (F/D>0.25) reflectors using simple unperturbed conic surfaces. Such a d
presents a requirement for the feed to efficiently illumninate the main reflector over a total subtended angle of typically 130 degrees. FIG. 1a illustrates one such design FIGS. 1b and 1c show models of the typical resulting amplitude and phase feed radiation patterns of this configuration.
In order to provide the larger angular illumination for a “deep dish” reflector (subtended angle >180 degrees), such a simple design is limited by internal and multi-path reflections prevalent within the cone structure between the rear reflecting surface and the leading edge boundary resulting in poorly controlled amplitude and phase radiation patterns with deep nulls at some frequencies within a typical operating band. FIG. 2a illustrates one such design. FIGS. 2b and 2c show typical models of the resulting amplitude and phase feed radiation patterns for this configuration.
Multiple internal reflections can be reduced by the use of a regular array of corrugations positioned on the leading edge (cone surface closest to the main reflector). FIG. 3a illustrates one such design. FIGS. 3b and 3c show typical models of the resulting amplitude and phase feed radiation patterns of this configuration, as described in European Patent Application 0 A439 800 A1 by Kuhne filed December 1990. Such a configuration improves the impedance match between the cone medium and that of free space, thus presenting a less severe impedance boundary to the RF signal path. However such a configuration only partially resolves the internal reflections and can have a detrimental effect on both amplitude and phase radiation match between E and H planes.
Therefore it is the object of the invention to provide an apparatus that overcomes limitations in the prior art, and in so doing present ea solution that allows such a feed design to provide reflector antenna characteristics which meet the most stringent electrical specifications over the entire operating band used for a typical terrestrial communication microwave link.